Embryonic cardiovascular systems regulate dramatic transitions in structure and function simultaneously. This process is critically important to achieve the complex 3D geometry and function of the mature heart and errors/failures in this process are one proposed pathogenetic mechanism for congenital cardiovascular malformations. This revised proposal represents an novel approach to define how specific changes in mechanical loading conditions alter regional CV contractility, sarcolemmal Ca2+ currents and myocyte passive and active properties. Specific Aim I. Define the relationship between mechanical wall strain, myocardial contractility, and Cab currents in stage 18 to 34 chick embryos during normal development, redistributed preload (LA ligation) or increased afterload (CT band) to define the time-course, location, and extent of functional adaptation that occurs in response to altered mechanical load. Hypothesis 1-1. Increased regional ventricular mechanical load measured as 2D and/or 3D epicardial strain increases local embryonic contractility measured as in vivo systolic wall stress- strain relations and contractility (Eav-max) and in vitro force-length, force-frequency, and force-velocity relations. Hypothesis 1-2. Increased mechanical load increases embryonic T-type and L-type Ca2+ currents as defined by simultaneous measurement of in vitro force-length relations and intracellular Ca2+ (strips); whole-cell patch clamp and Ca2+- imaging and Ni2+, mibefradil, nifedipine, and ryanodine blockade (strips and cells). Specific Aim 2. Define the relationship between mechanical wall strain, myofiber and collagen alignment, and viscoelastic properties in stage 18 to 34 chick embryos during normal development, redistributed preload (LA ligation) or increased afterload (CT band), complementing both in vivo and in vitro studies. Hypothesis 2-1. Increased ventricular mechanical load accelerates and redistributes myofiber alignment. Hypothesis 2-2. Increased ventricular mechanical load decreases compliance calculated from passive wall stress-strain relations (in vitro, finite element pseudo-strain energy density function) and increases collagen content and alignment. Significance: This revised proposal TESTS specific relationships between mechanical load, myocardial function, and myofiber and matrix architecture in the developing heart and provides a critical foundation for future experiments to identify the intracellular mechanisms (genes, proteins, trans-activating factors) that transduce mechanical forces in the developing myocardium.